In an exhaust emission control system of an engine, a three-way catalyst is disposed in an exhaust pipe and exhaust gas is cleaned by this three-way catalyst. When the air-fuel ratio of the exhaust gas is controlled to a stoichiometric air-fuel ratio, the three-way catalyst disposed in the exhaust pipe can efficiently clean the exhaust gas. For this reason, an air-fuel ratio sensor is disposed in the exhaust pipe and the air-fuel ratio of the exhaust gas is feedback controlled on the basis of the detected signal of the air-fuel ratio sensor, whereby the air-fuel ratio of the exhaust gas is controlled to the stoichiometric air-fuel ratio. However, the exhaust gas does not collide uniformly with the air-fuel ratio sensor, so it can be thought that the air-fuel ratio of only a specific cylinder is different from those of the other cylinders. In this case, there is a possibility that the air-fuel ratio of the specific cylinder is substantially different from an air-fuel ratio when the engine is viewed as a whole and hence deviates from the stoichiometric air-fuel ratio by a large amount.
For this reason, to control the air-fuel ratio with higher accuracy, a technology for detecting the air-fuel ratio of each cylinder has been known. For example, JP-A-2001-82221 discloses a technology for computing, by a crank angle, the timing in which the air-fuel ratio of each cylinder is detected, computing the air-fuel ratio of each cylinder on the basis of the air-fuel ratio detected at the timing, and feeding back the air-fuel ratio for each cylinder.
However, even if the detected air-fuel ratios of the cylinders are equal to each other, there is a case in which an air-fuel ratio detected by an air-fuel ratio sensor disposed in the exhaust collecting portion of an exhaust pipe is different from an actual air-fuel ratio, for example, because the shape of the exhaust pipe is different.
JP-A-2004-316483 (U.S. Pat. No. 6,830,042B2) discloses cylinder air-fuel ratio control for estimating the air-fuel ratios of a plurality of cylinders for the respective cylinders on the basis of the output of one air-fuel ratio sensor disposed in an exhaust collecting portion where exhaust gases from a plurality of cylinders merge with each other and controlling the air-fuel ratios of the plurality of cylinders (fuel injection quantities) for the respective cylinders.
Variations in the fuel injection quantity of each cylinder may be caused by the individual difference of a fuel injection system of each cylinder (variations in manufacture and time deterioration of an injection valve). Thus, even if the air-fuel ratios of the plurality of cylinders can be estimated with high accuracy for the respective cylinders on the basis of the output of the one air-fuel ratio sensor disposed in the exhaust collecting portion, variations in the air-fuel ratio between the cylinders may be increased by the individual differences of the fuel injection systems of the respective cylinders while the cylinder air-fuel ratio is performed. These variations in the air-fuel ratio between the cylinders become a cause of reducing the accuracy of the cylinder air-fuel ratio control to degrade emission and fuel consumption.
To reduce variations in the air-fuel ratio between cylinders of an internal combustion engine to improve the accuracy of air-fuel ratio control, a system is disclosed in Japanese Patent No. 3299120 (U.S. Pat. No. 5,947,095) and JP-A-2004-316483. In the system, a model of relating the detected value of an air-fuel ratio sensor disposed in an exhaust collecting portion of an internal combustion engine (the air-fuel ratio of the exhaust collecting portion) to a cylinder air-fuel ratio (air-fuel ratio of each cylinder) constructs an observer having a cylinder air-fuel ratio as a state variable, estimates the air-fuel ratio of each cylinder from the observation result of the observer, and corrects the fuel injection quantity of each cylinder according to the deviation between the estimated air-fuel ratio of each cylinder and a target air-fuel ratio, thereby making the air-fuel ratio of each cylinder equal to the target air-fuel ratio.
However, as described above, in the system for estimating the air-fuel ratio of each cylinder on the basis of the model of relating the detected value of the air-fuel ratio sensor to a cylinder air-fuel ratio (air-fuel ratio of each cylinder) as the state variable, as shown in FIG. 19A, when the air-fuel ratio of each cylinder is estimated, in the same manner as usual, at the time when the operating state of an internal combustion engine is suddenly changed, the air-fuel ratio of each cylinder in the present operating state is estimated by the use of a state variable in a different operating state in the past. Thus, the system has a shortcoming that the accuracy of estimating the air-fuel ratio of each cylinder may be remarkably reduced, which results in reducing the control accuracy of the air-fuel ratio of each cylinder.